The present invention pertains to a coal burner for a furnace in which pulverized coal of very fine sizes is utilized. Preferably, the maximum coal size is 40 microns, this size being somewhat similar to talcum powder. When coal exists in this size, especially if stored, there is a great tendency for the same to be self-igniting and therefore, it is common practice at present to connect a pulverizing unit directly with the burner, whereby the coal in pulverized form of the maximum size referred to above, is delivered to the burner and also actually is pulverized in an inert atmosphere, such as dry steam. Such steam is also the means by which the pulverized coal is blown under pressure into the furnace and at the inner end of the burner is ignited, due to being contacted with combustion air. Common types of pulverized coal burners presently in existence utilize air to transmit the coal to the burner and the burner carries the necessary air well mixed with the coal to the burner nozzle for ignition.
The feeding of fuel in dust or powdered condition to a combustion chamber by means of steam basically is old. U.S. Pat. No. 585,572, in the name of B. C. Heavey, dated June 29, 1897, discloses a furnace and burner in which this is accomplished.
U.S. Pat. No. 744,220, in the name of A. Neu, dated Nov. 17, 1903, discloses a fuel burner in which oil or ground coal is mixed with steam and air by a suitable apparatus for delivery to a furnace but the steam and fuel are not mixed until they reach the point of discharge into the furnace which is accomplished by means of a conical valve that spirals the fuel incident to being discharged.
U.S. Pat. No. 1,370,091, in the name of W. W. Conard, dated Mar. 1, 1921, discloses what appears to be a wheeled vehicle that utilizes pulverized fuel which is spiraled incident to feeding the same to a combustion chamber by means of nozzles.
More recently, U.S. Pat. No. 3,299,841, in the name of F. L. Hemker et al, dated Jan. 24, 1967, discloses a burner for a furnace in which powdered coal mixed with air is blown through a burner nozzle and at the delivery end of said nozzle, the mixture is expanded by a series of frusto-conical impellers on the delivery end of a concentric impeller tube that is mounted within the burner tube. Secondary air is mixed with the coal and primary air by means of a circular pattern of registers surrounding the burner nozzle, but there is no disclosure of the powdered coal being conveyed to the combustion zone by an inert gaseous vehicle.
When coal is transmitted to the combustion zone of a furnace by means of an inert gas, such as steam, the air required for ignition must be added to the mixture of coal and inert vehicle at the discharge end of the burner which is immediately adjacent the combustion zone in the furnace. In accordance with present invention, testing has proved that the ratio of coal to inert atmosphere or vehicle under such circumstances is critical to ignition, especially when steam is used to transmit coal in the very fine size range of 40 microns and finer. More specifically, it has been proven that the maximum ratio of approximately 2.5 pounds of steam to 1 pound of coal is required for clean burning of the fuel. It also has been found that the coal will burn even more effectively when the steam is reduced in this ratio where the coal is increased. The present invention is designed to either increase the coal or reduce the inert gas in the area of the burner where ignition is initiated. Details of such design are set forth below.